1. Field of the Invention
This invention relates to an igniter plug which is exposed to the presence of extremely great temperature difference, and particular concerned to an igniter plug for use in a rocket propelled engine in which liquefied hydrogen and liquefied oxygen are employed as components of liquid fuel.
2. Description of the Prior Art
In an igniter plug for use in a rocket propelled engine in which liquid fuel mixture is ignited at combustion chamber to generate a propulsion, the igniter includes a metallic shell into which a tubular insulator is placed through which a center electrode passes.
In this instance, a front end of the insulator somewhat extends outside beyond that of the metallic shell.
Meanwhile, the plug is exposed to very low temperature such as approximately 200 degrees Celsius below the freezing point when the engine just began to start.
Once the engine has started, the plug is exposed to extremely high temperature such as some thousands degrees Celsius. In addition to this, aqueous component emerged at the time of the fuel combustion, adheres to an extended front end, and retained there.
The aqueous component is frozen by the liquefied fuel at the time of restarting the engine. This freezing hinders temperature of the insulator from uniformly rising.
The huge temperature difference causes thermal shock to run through the insulator, at the same time, the freezing causes to induce thermal stress at the insulator to eventually result in cracks. Because the prior insulator has been made of an alumina ceramic material having a poor thermal conductivity at high temperature, and a relatively great thermal expansional coefficient of 8.5-10.0.times.10.sup.-6 /k.
Therefore, it is an object of this invention to provide the igniter plug which is capable of avoiding thermal shock from being induced, and at the same time, substantially preventing aqueous component from adhering, and being retained to an insulator thereby making free from freeze of the aqueous component, relieving of thermal stress to occur.
According to the invention there is provided the igniter plug comprising; the cylindrical metallic shell; the tubular insulator concentrically located within the metallic shell; the center electrode placed to pass through an inner bore of the insulator, the front end of the electrode extending outside beyond the front end surface of the insulator to be exposed to a passage through which an liquid fuel mixture passes; the insulator being made from a sintered ceramics which includes silicone nitride component ranging from 85% to 99% inclusive by weight, while the front end surface of the insulator being retracted into that of the metallic shell, retracted degree of the insulator being within the range from 1.5 mm to 2.5 mm.
The insulator is made of sintered ceramics including silicon nitrate component ranging from 85% to 99% by weight. The sintered ceramics has a small thermal coefficient of 3.0.times.10.sup.-6 -3.5.times.10.sup.-6 /K with a relatively good thermal conductivity, thus protecting thermal shock from being induced on the insulator to prevent cracks even though the igniter is exposed to an atmosphere of huge temperature difference.
The reason the weight percent of the silicon nitrate component is determined as above, is that the silicon nitride under the level of 85%, becomes short of bending strength, while the one exceeding to the level of 99%, becomes poor in sintering and rigidity.
On the other hand, the front end of the insulator is retracted inner side from that of the metallic shell by the degree from 1.5 mm to 2.5 mm, thus avoiding aqueous component from adhering, and being retained to the front end of the insulator.
This holds the insulator free from freezing by preventing thermal stress from occurring due to uniform temperature rise, avoiding cracks from occurring on the insulator.
These and other objects and advantages of the invention will be apparent upon reference to the following specification, attendant claims, and drawings.